The invention relates to an arrangement for erasably and continually recording the value of a measured quantity on a record carrier of the type which can be written upon by means of magnetic fields. The invention is especially concerned with record carriers of the type on which a visible, legible record can be made during relative movement between the writing unit of a recording apparatus, on the one hand, and the record carrier, on the other hand.
In many situations where it is desired to continually record the value of a continually measured quantity, it is not actually desired to keep a record of the entire history of the value of that quantity. Instead, it is many times only of interest to know about the variation in the value of the measured quantity for a limited period of time immediately preceding an event of interest, with the history of the measured quantity prior to such limited period of time being of no interest whatsoever. To this end, it is possible to continually record the measured value and, simultaneously therewith, continually erase that portion of the record corresponding to times which are no longer of interest. However, if in this way a record is continually formed and continually erased, with only a portion of the record corresponding to a time interval of preselected duration being in existence at any given moment, then especially when the time interval is of short duration there can arise serious difficulties as to resolution and legibility of the record.
An example of a situation of the type in question is the recording of vehicle operation data. It is often the case that such records are automatically made for the sole purpose of having reliable evidence of the performance of the vehicle during a relatively short time interval preceding a vehicular accident. For example, it may be important to know the speed of a truck for a one-minute time interval preceding a collision in which the truck is involved, for the purpose of proving the presence or absence of negligence, without there being any interest in the speed record of the truck during the hours preceding such collision. In that case, the speed of the truck is continually measured and recorded, but also continually erased, with only the portion of the speed record corresponding to the most recently travelled minute being in existence at any given moment.
It will be understood that considerable demands are made upon devices designed for the collection of information relating to vehicular accidents, because of the need for unquestionable accuracy, legibility, permanence, unequivocal interpretability, and so forth. The continually formed record must be of satisfactory quality over a wide range of temperature and humidity conditions certain to be encountered during use. Additionally, the record preferably should be directly legible -- i.e., without the need for reading transducers or developing processes -- so that the record can be easily interpreted using the naked eye. Likewise, the record carrier should be of such a character that a traffic officer arriving at the scene of the accident can readily remove the record carrier from the recording apparatus, so that the record may serve as part of the evidence which he collects. Because the record carrier in such a situation may be handled roughly, it is advantageous that the recorded information thereon be fixedly recorded, hold fast and not be rubbed off during handling nor be detrimentally affected by the frictional forces which can develop when it is rubbed. Furthermore, the record carrier advantageously should be both usable as an original and also easily copied.
These requirements are met by a recently disclosed paper-like recording material whose visual characteristics can be altered by magnetic means. This recording material is essentially comprised of two transparent or translucent cover layers and an intermediate emulsion. The emulsion is comprised of oil-filled microcapsules in which are suspended metallic microplates. The microplates are of magnetic material and reflect light. When the recording material is in blank condition, these magnetic microplates are oriented generally in the plane of the record carrier and reflect the light incident upon the record carrier. As a result, the record carrier appears relatively light in color. If now a permanent-magnet or electromagnet writing element is moved over the surface of the record carrier, not necessarily in actual contact therewith, there develops along the path of movement a visible recording trace. The recording trace is dark in color compared to the surrounding surface of the record carrier, and the microplates of the record carrier along the recording trace are found to have been turned by 90.degree. from their original position so as to be oriented generally normal to the surface of the record carrier. As a result, light incident upon the recording trace will be transmitted and/or absorbed by the intermediate emulsion layer, so that the recording trace will appear dark in color.
For such a record carrier, the function of an appropriate erasing device is to first stir around the magnetic microplates in the record carrier and finally turn them into positions lying in the plane of the record carrier, so that incident light will be reflected and the record carrier appear light in color. Experience has shown that an optimal erasing effect is achieved by using two magnetic fields, preferably two alternating magnetic fields, both operative in the plane of the record carrier. In such case, lines extending in the directions of effectiveness of the magnetic fields are crossed by a line extending in the direction of the requisite relative movement between the record carrier and the erasing device generating the magnetic fields. During this relative movement, the magnetic fields at each portion of the record carrier to be erased are operative in directions generally perpendicular to each other and/or they are operative in alternation.
These required characteristics for the erasing device make necessary the use of relatively expensive erasing apparatuses incorporating in general a plurality of coils, in addition to whatever may be required for the generation of the erasing fields proper. For example, it is very desirable to make use of disk-shaped record carriers because, compared to strip-shaped record carriers, especially long ones, they are much easier to handle. However, disk-shaped record carriers require the use of specially shaped field-generating coils for erasing device, and these specially shaped coils can be produced on a mass-production basis only with difficulty.